1. Field of the Invention
The present invention relates generally to non-destructive evaluation, and more particularly to a method and apparatus for laser ultrasound inspection of materials with synthetic aperture focusing.
2. Description of the Related Art
Laser ultrasound involves the generation or detection of ultrasound in materials with lasers. Generally, in laser ultrasound, a source laser irradiates a material with a laser beam along its surface. Ultrasonic waves are generated by the laser beam by non-destructive local heating of the surface to create expansion and a strain wave, or by increasing the amplitude of the beam to vaporize a small amount of material to form a plasma that strikes the surface like a hammer.
The ultrasonic wave propagates throughout the material and is reflected back to the surface of the material. As the reflected ultrasonic wave returns to the surface of the material, typically an interferometric detector is used to detect either displacement or velocity at the surface by irradiation of the surface with another laser beam. The detected displacement or velocity signals are used to generate an image of the material. A more detailed discussion on laser ultrasound is provided in C. B. Scruby et al., Laser Ultrasonics-Techniques and Applications (IOP Publishing Ltd. 1990), which is hereby incorporated by reference.
When laser ultrasound is used to inspect thin structures, image resolution generally decreases because Lamb waves are generated. Lamb waves are generated when the object being imaged is less than several wavelengths thick, as surface waves engage the thin structure on its opposing face. Lamb waves are dispersive, which means that different frequency components of the Lamb wave propagate with different velocities. The dispersive characteristic of Lamb waves results in decreased image resolution, since the arrival time of the reflected wave is less well defined.
Although it is possible to compensate for frequency dispersion in Lamb waves through modeling, this technique involves considerable analytical effort. In addition, where multiple modes of propagation exist, it is analytically difficult to model all modes accurately. These problems are compounded in anisotropic materials in which the wave speed is dependent on propagation direction.
It would be desirable, therefore, to have a method and apparatus for accurately and efficiently imaging a thin structure using laser ultrasound.